JP2024509806A - A porous carbon material from which impurities have been removed, a method for producing the same, a positive electrode for a lithium-sulfur battery containing the carbon material as a positive electrode active material, and a lithium-sulfur battery - Google Patents

Abstract

A porous carbon material from which impurities such as moisture have been removed through pretreatment can be applied to a positive electrode for a lithium-sulfur battery to improve the charging overvoltage problem of a battery. This manufacturing method, a positive electrode for a lithium-sulfur battery, and a lithium-sulfur battery including the carbon material as a positive electrode active material are disclosed. The porous carbon material has a specific surface area of 200 to 1,700 m2/g, and is characterized in that impurities are removed through pretreatment using microwaves.

Description

This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0180557 dated December 16, 2021 and Korean Patent Application No. 10-2022-0152348 dated November 15, 2022, and the corresponding Korean patent application All contents disclosed in the literature are incorporated herein by reference.

The present invention relates to a porous carbon material from which impurities have been removed, a method for producing the same, a positive electrode for a lithium-sulfur battery, and a lithium-sulfur battery containing the carbon material as a positive electrode active material. The porous carbon material from which impurities have been removed can be applied to a positive electrode for lithium-sulfur batteries to improve the charging overvoltage problem of batteries. The present invention relates to a positive electrode for a lithium-sulfur battery and a lithium-sulfur battery containing a positive electrode active material.

As interest in energy storage technology increases, the field of application is expanding to include energy for mobile phones, tablets, laptops, camcorders, and even electric vehicles (EVs) and hybrid electric vehicles (HEVs). Research and development on electrochemical devices is gradually increasing. Electrochemical devices are the field that is attracting the most attention in this aspect, and among them, the development of secondary batteries such as rechargeable and dischargeable lithium-sulfur batteries has been the focus of attention, and recently there has been a Batteries development has led to research and development into new electrode and battery designs to improve capacity density and specific energy.

Among these electrochemical devices, lithium-sulfur batteries (Li-S batteries) have high energy density (theoretical capacity) and are attracting attention as next-generation secondary batteries that can replace lithium-ion batteries. There is. In such a lithium-sulfur battery, a reduction reaction of sulfur and an oxidation reaction of lithium metal occur during discharge, and at this time, sulfur is converted into lithium polysulfide (LiPS), which has a linear structure, rather than _S8 , which has a ring structure. Such a lithium-sulfur battery is characterized by a gradual discharge voltage until the polysulfide is completely reduced to _Li2S .

However, the biggest obstacle in the commercialization of lithium-sulfur batteries is the battery life.During the charging/discharging process, the charging/discharging efficiency decreases and the battery life deteriorates. The causes of this deterioration in the life of lithium-sulfur batteries include side reactions of the electrolyte (deposition of byproducts due to decomposition of the electrolyte), instability of lithium metal (dendrites grow on the lithium negative electrode, causing short circuits), and cathode damage. There are various causes such as by-product deposition (lithium polysulfide seepage from the positive electrode).

In other words, in a battery that uses a sulfur-based compound as a positive electrode active material and an alkali metal such as lithium as a negative electrode active material, lithium polysulfide gushes out and shuttles phenomenon occurs during charging and discharging, and lithium polysulfide becomes a negative electrode active material. As a result, the capacity of the lithium-sulfur battery is reduced, and the lithium-sulfur battery has major problems in that its lifespan is shortened and its reactivity is reduced. That is, since the polysulfides expelled from the positive electrode have high solubility in the organic electrolyte, unwanted movement (PS shuttling) may occur through the electrolyte toward the negative electrode, resulting in irreversible loss of the positive electrode active material. This results in a decrease in capacity due to oxidation, and a decrease in battery life due to deposition of sulfur particles on the lithium metal surface due to side reactions.

On the other hand, in order for lithium-sulfur batteries to have a high energy density of about 400 Wh/kg or more or 600 Wh/L or more, high loading (about 4.0 mAh/cm2 ^or more) and low porosity (about 60% or less) are required. An electrolyte and cathode active material system that can be operated under these conditions is required. In other words, the behavior of such a lithium-sulfur battery can vary greatly depending on the electrolyte, and when the sulfur in the positive electrode is leached into the electrolyte in the form of lithium polysulfide (LiPS), the electrolyte is a catholyte. An electrolyte in which almost no sulfur is extracted in the form of lithium polysulfide is called SSE (sparingly solvating electrolyte). Lithium-sulfur batteries that utilize existing catholyte systems rely on liquid-phase reactions through the production of intermediate polysulfide in the form of Li _{2 S} The problem is that the discharge capacity (1,675 mAh/g) cannot be fully utilized, and the life of the battery is rapidly reduced due to deterioration of the battery due to the outflow of polysulfide.

On the other ^hand , recently, an SSE (sparingly solvating electrolyte) electrolyte system that can suppress the outflow of polysulfides has been developed. It was confirmed that more than 90% of the theoretical discharge capacity of sulfur can be utilized when applied as a carrier. However, in the case of such carbon materials, because of their high specific surface area, they also contain a relatively large amount of impurities such as water, which increases electrode side reactions and causes charging overvoltage, which reduces the efficiency of use. There's a problem. To solve this problem, in the art, carbon materials with a high specific surface area are heat treated in a furnace, but this takes a long time and makes it difficult to effectively remove impurities.

Therefore, there is a need for a method that can improve the charging overvoltage problem by effectively removing impurities such as water contained in a carbon material having a high specific surface area while using an SSE electrolyte system.

Therefore, an object of the present invention is to improve the charging overvoltage problem of a lithium-sulfur battery by applying a porous carbon material from which impurities such as moisture have been removed through pretreatment to a positive electrode for a lithium-sulfur battery. The object of the present invention is to provide a porous carbon material that has been removed, a method for producing the same, a positive electrode for a lithium-sulfur battery, and a lithium-sulfur battery containing the carbon material as a positive electrode active material.

To achieve the above object, the present invention provides a porous carbon material having a specific surface area of 200 to 1,700 m ² /g and having impurities removed through pretreatment using microwaves.

The present invention also provides the steps of (a) putting a porous carbon material having a specific surface area of 200 to 1,700 m ² /g into a closed container and then purging it by injecting an inert gas; Provided is a method for producing a porous carbon material from which impurities are removed, including the step of applying microwaves to the porous carbon material.

Further, the present invention provides a positive electrode for a lithium-sulfur battery, which includes, as a positive electrode active material, a sulfur-carbon composite in which sulfur is supported on a porous carbon material from which the impurities have been removed.

The present invention also provides a positive electrode for the lithium-sulfur battery; a negative electrode; a separation membrane interposed between these; and a first solvent containing a fluorine-based ether compound, a second solvent containing a glyme-based compound, and a lithium salt. A lithium-sulfur battery is provided including: an electrolyte;

According to the porous carbon material from which impurities are removed, the manufacturing method thereof, the positive electrode for a lithium-sulfur battery, and the lithium-sulfur battery containing the carbon material as a positive electrode active material according to the present invention, impurities such as moisture are removed through pretreatment. By applying this porous carbon material to the positive electrode for lithium-sulfur batteries, it is possible to improve the charging overvoltage problem of batteries. In addition, the positive electrode for lithium-sulfur batteries, which includes a porous carbon material from which the impurities have been removed as a positive electrode active material, is a cathode that is not an existing catholyte electrolyte system (discharge capacity: ~1,200 mAh/gs). ) By combining with an electrolyte system (discharge capacity ~ 1,600mAh/gs), more than 90% of the theoretical discharge capacity of sulfur (1,675mAh/g) can be utilized, and the energy density is approximately 400Wh/kg or more. Or, it has the advantage of being maintained at a high level of 600Wh/L or more.

2 is a TGA analysis graph for confirming whether impurities can be removed from porous carbon materials according to an example of the present invention and a comparative example. 2 is a graph showing a temperature profile over time when a porous carbon material and a sulfur-carbon composite are irradiated with microwaves under the same conditions. 2 is a TGA analysis graph showing the degree of weight loss due to temperature increase obtained by performing TGA analysis on a porous carbon material according to an embodiment of the present invention and a sulfur-carbon composite according to a comparative example.

The present invention will be explained in detail below.

The porous carbon material according to the present invention has a specific surface area of 200 to 1,700 m ² /g, and is characterized by having impurities removed through pretreatment using microwaves.

In order for a lithium-sulfur battery to achieve a high energy density of approximately 400 Wh/kg or more or 600 Wh/L or more, it is necessary to use high loading (approximately 4.0 mAh/ ^cm2 or more) and low porosity (approximately 60% or less) conditions. A drivable electrolyte and cathode active material system is required. This electrolyte system compensates for the problem of electrolyte (catholyte), where sulfur from the positive electrode is extracted into the electrolyte in the form of lithium polysulfide (LiPS). A sparingly solvating electrolyte (SSE) electrolyte system has been developed that can suppress seepage. In addition, if a carbon material with a particularly high specific surface area (BET Specific surface area) of 200 to 1,700 m ² /g is applied as a positive electrode active material as a sulfur support, and this is combined with an SSE electrolyte system, sulfur It was confirmed that more than 90% of the theoretical discharge capacity can be utilized (that is, the sulfur utilization rate is 90% or more, and the above "sulfur utilization rate" specifically refers to the theoretical capacity per weight of sulfur. The ratio of discharge capacity (mAh) per weight (gram) of sulfur element contained in the positive electrode of the relevant battery compared to 1,675mAh/g, for example, the weight of sulfur element present in the positive electrode of a lithium-sulfur battery. When the per discharge capacity is 1,600mAh/g, the sulfur utilization rate is 95.5% (1,600/1,675)).

However, in the case of such carbon materials, impurities such as moisture due to their high specific surface area (specifically, moisture contained in the carbon materials and other impurities such as unnecessary functional groups existing inside and on the surface of the carbon materials) Since it is also contained in a relatively large amount, electrode side reactions become large and a charging overvoltage phenomenon occurs, resulting in a problem of low utilization. To solve this problem, in the art, carbon materials with high specific surface areas are heat treated in a furnace, but this takes a long time and makes it difficult to effectively remove impurities.

Therefore, while using the SSE electrolyte system, the applicant has developed a carbon material with a high specific surface area by effectively removing impurities such as moisture contained in the carbon material through pretreatment using microwaves. Increased utilization of carbon materials. In addition, by improving the ability of the carbon material to support sulfur, it became possible to manufacture a uniform positive electrode active material, thereby improving the charging overvoltage problem.

As in the present invention, if a porous carbon material with a high specific surface area is pre-treated by applying (or irradiating) microwaves, it can be quickly processed by energy transfer rather than by conventional heat transfer. Temperature increases and interruptions are possible. In particular, because the energy input into the porous carbon material is converted into thermal energy at a high rate, it takes less time to remove impurities, making it more efficient than existing methods.

On the other hand, the present invention is to remove impurities contained in the carbon material by applying microwaves to the porous carbon material, but when the porous carbon material and another substance (for example, sulfur) are combined If microwaves are applied in a carbonized state, it is impossible to selectively remove impurities contained in the carbon material. For example, when microwaves are applied to a sulfur-carbon composite made of a porous carbon material and sulfur, the sulfur is also vaporized and volatilized, so it is difficult to remove only the impurities contained in the carbon material. It is possible. This is because when microwaves are applied, the rate of temperature rise of carbon is faster than that of sulfur, and the energy generated when the temperature of carbon rises is transferred to the surrounding sulfur, and at this time impurities contained in carbon are This is because sulfur is vaporized at low temperatures before being removed.

As described above, the porous carbon material of the present invention has a specific surface area of 200 to 1,700 m ² /g, and is characterized by having impurities removed through pretreatment using microwaves. When the porous carbon material is used in fields other than the battery field, there are no particular restrictions even if the specific surface area falls outside the above range as long as impurities are removed by pretreatment using microwaves. However, in lithium-sulfur batteries that utilize an SSE electrolyte system to utilize more than 90% of the theoretical discharge capacity of sulfur, using a carbon material with a specific surface area within the above range maximizes battery performance. can be done. Further, the porous carbon material may have a pore volume of 1.5 cm ³ /g or more. If the pore volume of the porous carbon material is less than 1.5 cm ³ /g, the amount of sulfur supported may be reduced, making it difficult to realize a high energy density battery.

Examples of carbon materials to which microwaves are applied include carbon nanotubes; graphene (especially multilayer graphene flake, MGF); graphite; carbon black, acetylene black, Ketjen black, Denka black, thermal black, and channel black. , carbon black such as furnace black and lamp black; carbon fiber; or a mixture containing two or more of these.

Further, the pretreatment using microwaves is characterized in that microwaves are applied to the porous carbon material under the condition that MPPT is 2,000 to 10,000 W*s/g according to Equation 1 below.

[Formula 1]
MPPT (W*s/g) = Microwave Power (W) x Time (s)/Carbon material mass (g)

In Equation 1, the time (Times) exceeds 10 seconds as the time in seconds during which the microwave is applied. Preferably, the porous carbon material is in powder form.

However, each carbon material before microwave application differs in the content of other impurities such as water and functional groups other than carbon, so the MPPT (W*s/g) value in Formula 1 above also differs. It is.

In one embodiment, the carbon nanotubes have an MPPT value of 2,000 to 10,000 W*s/g, and if the MPPT value exceeds about 2,800 W*s/g, no further impurity removal is performed. Therefore, there may be no real benefit. If the MPPT value of the carbon nanotube is less than 2,000 W*s/g, it is impossible or insufficient to remove impurities, making it difficult to achieve the purpose of the present invention, which is to improve charging overvoltage. If the MPPT value of the carbon nanotube exceeds 10,000 W*s/g, it may lead to ignition.

The MPPT value of the graphene (especially multilayer graphene flake, MGF) is also 2,000 to 10,000 W*s/g, but if it exceeds about 5,300 W*s/g, it is difficult to remove impurities further. There may be no real benefit because it is not carried out. If the MPPT value of the graphene is less than 2,000 W*s/g, impurity removal is impossible or insufficient, making it difficult to achieve the purpose of the present invention, which is to improve charging overvoltage. If the MPPT value of graphene exceeds 10,000 W*s/g, it may lead to ignition.

The MPPT value of the carbon black is also 2,000 to 10,000 W*s/g, but if it exceeds about 3,500 W*s/g, there is no practical benefit because impurities are not removed any further. There is. If the MPPT value of the carbon black is less than 2,000 W*s/g, it is impossible or insufficient to remove impurities, making it difficult to achieve the purpose of the present invention, which is to improve charging overvoltage. If the MPPT value of the carbon black exceeds 10,000 W*s/g, it may lead to ignition.

The Ketjen black has an MPPT value of 2,000 to 10,000 W*s/g, preferably 5,000 to 9,900 W*s/g, and when it exceeds 9,900 W*s/g, There may be no practical benefit as impurities are not removed any further. If the MPPT value of the Ketjen black is less than 2,000 W*s/g, it is impossible or insufficient to remove impurities, making it difficult to achieve the purpose of the present invention, which is to improve charging overvoltage. If the MPPT value of the Ketjenblack exceeds 10,000 W*s/g, it may lead to ignition. Therefore, when applying microwaves to carbon materials, microwave energy must be applied in accordance with the range of MPPT applicable to each carbon material.

The porous carbon material to which microwaves are applied having the specific surface area and pore volume as described above contains impurities of 90 to 100%, preferably 99 to 100% of the total impurities contained in the carbon material. is characterized in that it has been removed. The impurities include water, and specifically include water contained in the porous carbon material and other impurities such as unnecessary functional groups present inside and on the carbon material. On the other hand, even if impurities are removed by applying microwaves to the carbon material having a high specific surface area as described above, it is inevitable that moisture will be reabsorbed during storage.

On the other hand, if a porous carbon material with a high specific surface area is pretreated using microwaves in conjunction with the present invention (in other words, the sulfur-carbon composite contained in the positive electrode active material of a lithium-sulfur battery) By subjecting only the carbon material to microwave treatment before manufacturing the battery, the amount of sulfur that actually participates in the reaction during operation of the lithium-sulfur battery can be controlled more quickly and accurately than before. For example, when sulfur and carbon material without microwave pretreatment are mixed at a weight ratio of 70:30, if impurities are contained in the carbon material at a content of 5% by weight, the actual weight ratio of sulfur and carbon material is It becomes 70:28.5 (that is, 70:(30×0.95)). In other words, the discharge capacity of a lithium-sulfur battery is calculated based on the content of sulfur contained in the battery, but as in the present invention, carbon material with a high specific surface area is pretreated using microwaves. In this way, the content of sulfur and carbon material contained in the positive electrode active material can be more accurately determined. In other words, there is no impurity in the porous carbon material, and there is no error in the content of sulfur and carbon material contained in the positive electrode active material.

Next, a method for manufacturing a porous carbon material from which the impurities described above are removed will be described. The method for producing a porous carbon material from which impurities have been removed includes (a) placing a porous carbon material having a specific surface area of 200 to 1,700 m ² /g in a closed container, and then injecting an inert gas; and (b) applying microwaves to the porous carbon material.

Examples of the airtight container include ordinary containers such as glass jars that can be sealed and purged even when an inert gas is injected therein. The inert gas is a general inert gas such as nitrogen ( _N2 ), and there are no particular restrictions on the conditions for injecting the inert gas. The step (b) is characterized in that microwaves are applied to the porous carbon material under the condition that MPPT is 2,000 to 10,000 W*s/g according to Equation 1 below.

[Formula 1]
MPPT (W*s/g) = Microwave Power (W) x Time (s)/Carbon material mass (g)

In Equation 1, the time (Times) exceeds 10 seconds as the time in seconds during which the microwave is applied. Preferably, the porous carbon material is in powder form.

However, each carbon material before microwave application differs in the content of other impurities such as water and functional groups other than carbon, so the MPPT (W*s/g) value in Formula 1 above also differs. There are things to do. The explanation regarding this is an alternative to that described above as one embodiment.

Next, a positive electrode for a lithium-sulfur battery according to the present invention will be explained. The positive electrode for a lithium-sulfur battery includes, as a positive electrode active material, a sulfur-carbon composite in which sulfur is supported on a porous carbon material from which the impurities have been removed.

The positive electrode for a lithium-sulfur battery includes a positive active material, a binder, a conductive material, and the like. The positive electrode active material may include elemental sulfur (S ₈ ), a sulfur-based compound, or a mixture thereof, in addition to the porous carbon material from which impurities have been removed, as described above. Specifically, the compound may be Li _{2 Sn} (n≧1) or an organic sulfur compound. As explained above, a sulfur-carbon composite ((C ₂ S _x ) _n : x=2.5 to 50, n ≧2) is preferably used as the positive electrode active material.

The sulfur-carbon composite may have a particle size of 1 to 100 μm. If the particle size of the sulfur-carbon composite is less than 1 μm, the resistance between the particles may increase and overvoltage may occur at the electrode of the lithium-sulfur battery, and if it exceeds 100 μm, the As the surface area decreases, the wetting area with the electrolyte in the electrode and the reaction sites with lithium ions decrease, and the amount of electron transfer decreases relative to the size of the composite. Due to the slow reaction, the discharge capacity of the battery may be reduced.

The sulfur (S) may be included in an amount of 60 to 90% by weight, preferably 65 to 85% by weight, and more preferably 65 to 80% by weight based on the total weight of the positive active material. If sulfur is used in an amount less than 60% by weight based on the total weight of the positive electrode, a problem may occur in which the energy density of the battery is reduced; if sulfur is used in an amount exceeding 90% by weight, the electrode Problems may arise in which the conductivity within the electrode decreases and the safety of the electrode decreases.

The above positive electrode active material containing sulfur and carbon material may be included in an amount of 80 to 99 parts by weight, preferably 90 to 95 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the content of the positive electrode active material is less than 80 parts by weight based on 100 parts by weight of the total weight of the positive electrode, a problem may occur in which the energy density of the battery decreases, and if it exceeds 99 parts by weight, the electrode The problem may occur that the conductivity inside the electrode decreases and the stability of the electrode decreases.

The binder contained in the positive electrode is a component that assists in bonding the positive electrode active material to the conductive material and to the current collector, such as polyvinylidene fluoride (PVdF), polyvinylidene fluoride-polyhexafluoropropylene copolymer (PVdF), etc. /HFP), polyvinyl acetate, polyvinyl alcohol, polyvinyl ether, polyethylene, polyethylene oxide, alkylated polyethylene oxide, polypropylene, polymethyl (meth) acrylate, polyethyl (meth) acrylate, polytetrafluoroethylene (PTFE), polyvinyl chloride, poly Acrylonitrile, polyvinylpyridine, polyvinylpyrrolidone, styrene-butadiene rubber, acrylonitrile-butadiene rubber, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM rubber, styrene-butylene rubber, fluororubber, carboxymethyl cellulose (CMC), starch, hydroxy One or more types selected from the group consisting of propyl cellulose, regenerated cellulose, and mixtures thereof can be used, but the cellulose is not necessarily limited thereto.

The binder is generally added in an amount of 1 to 50 parts by weight, preferably 3 to 15 parts by weight, based on 100 parts by weight of the total weight of the positive electrode. If the content of the binder is less than 1 part by weight, the adhesive force between the positive electrode active material and the current collector may be insufficient; if it exceeds 50 parts by weight, although the adhesive strength is improved, the positive electrode active material is The content of the substance may be reduced and the battery capacity may be lowered.

The conductive material contained in the positive electrode is not particularly limited as long as it has excellent electrical conductivity without causing side reactions in the internal environment of the battery and without causing chemical changes in the battery, and typically includes Graphite or conductive carbon can be used, for example graphite such as natural graphite, artificial graphite; carbon such as carbon black, acetylene black, Ketjen black, Denka black, thermal black, channel black, furnace black, lamp black, etc. Black: Carbon-based materials whose crystal structure is graphene or graphite; Carbon nanotubes; Conductive fibers such as carbon fibers and metal fibers; Carbon fluoride; Metal powders such as aluminum powder and nickel powder; Conductive whiskers; conductive oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives; can be used alone or in combination of two or more, but are not necessarily limited to this. .

The conductive material is generally added in an amount of 0.5 to 10 parts by weight, preferably 0.5 to 5 parts by weight, based on 100 parts by weight of the entire positive electrode, but it is not included in the positive electrode of the present invention. Sometimes it doesn't work. If the content of the conductive material is too large, exceeding 10 parts by weight, the amount of the positive electrode active material may become relatively small, resulting in a decrease in capacity and energy density. The method of incorporating the conductive material into the positive electrode is not particularly limited, and any conventional method known in the art, such as coating the positive electrode active material, can be used. Further, if necessary, the addition of a conductive material as described above can be replaced by adding a conductive second coating layer to the positive electrode active material.

Further, a filler can be selectively added to the positive electrode of the present invention as a component for suppressing expansion thereof. Such fillers are not particularly limited as long as they can suppress the expansion of the electrodes without causing chemical changes in the battery, and include, for example, olefin polymers such as polyethylene and polypropylene; glass fibers, and carbon. Fibrous substances such as fibers; etc. can be used.

The positive electrode is manufactured by dispersing and mixing a positive electrode active material, a binder, a conductive material, etc. in a dispersion medium (solvent) to create a slurry, and coating the slurry on a positive electrode current collector, followed by drying and rolling. can be done. The dispersion medium may include NMP (N-methyl-2-pyrrolidone), DMF (dimethyl formamide), DMSO (dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof, but is not limited thereto. It's not something you can do.

The positive electrode current collector includes platinum (Pt), gold (Au), palladium (Pd), iridium (Ir), silver (Ag), ruthenium (Ru), nickel (Ni), stainless steel (STS), and aluminum. (Al), molybdenum (Mo), chromium (Cr), carbon (C), titanium (Ti), tungsten (W), ITO (In doped SnO ₂ ), FTO (F doped SnO ₂ ), and alloys thereof. , aluminum (Al) or stainless steel whose surface is treated with carbon (C), nickel (Ni), titanium (Ti), or silver (Ag) can be used, but it is certainly limited to these. It's not a thing. The positive electrode current collector may be in the form of a foil, film, sheet, punched object, porous body, foam, or the like.

Finally, the lithium-sulfur battery according to the present invention will be explained. The lithium-sulfur battery includes the above-described positive electrode for lithium-sulfur battery, a negative electrode, a separation membrane interposed between them, a first solvent containing a fluorine-based ether compound, a second solvent containing a glyme-based compound, and lithium. Contains electrolytes including salts.

The lithium-sulfur battery of the present invention utilizes an SSE (sparingly solvating electrolyte) electrolyte system, but has a high specific surface area (BET specific surface area) of 200 to 1,700 m ² /g and at the same time microwave is applied. The cathode active material contains a porous carbon material from which impurities have been removed, and can utilize 90% or more, preferably 94 to 100%, of the theoretical discharge capacity of sulfur, and at the same time has a discharge capacity of approximately 400Wh/kg or more. It has a high energy density of 600Wh/L or more.

Hereinafter, the first solvent containing a fluorine-based ether compound, the second solvent containing a glyme-based compound, and the lithium salt contained in the electrolyte of the lithium-sulfur battery according to the present invention will be specifically explained.

The first solvent is an electrolyte solvent containing a fluorine-based ether compound, and has the effect of dissolving polysulfide and inhibiting solvent decomposition, thereby improving the coulombic efficiency (C.E.) of the battery, etc. plays a role in improving battery life. More specifically, the first solvent containing the fluorine-based ether compound has excellent structural stability as compared to general organic solvents containing alkenes due to fluorine substitution, and therefore has very high stability. Therefore, if this is used as an electrolyte of a lithium-sulfur battery, the stability of the electrolyte can be greatly improved, thereby improving the life performance of the lithium-sulfur battery.

Examples of the fluorine-based ether compounds include 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (1,1,2,2-tetrafluoroethyl2,2,3,3- tetrafluoropropyl ether, TTE), bis(fluoromethyl) ether, 2-fluoromethyl ether, bis(2,2,2-trifluoroethyl) ether, propyl 1,1,2,2-tetrafluoroethyl ether, isopropyl 1, 1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl isobutyl ether, 1,1,2,3,3,3-hexafluoropropylethyl ether, 1H,1H,2'H , 3H-decafluorodipropyl ether, and 1H, 1H, 2'H-perfluorodipropyl ether.

The second solvent is an electrolyte solvent containing a glyme-based compound (but not fluorine), which not only dissolves lithium salt so that the electrolyte has lithium ion conductivity, but also serves as a positive electrode active material. Its role is to generate sulfur and facilitate the electrochemical reaction with lithium.

Specific examples of the glyme-based compounds include dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, and triethylene glycol methyl. One or more selected from the group consisting of ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether can be mentioned. However, it is not limited thereto, and among these, dimethoxyethane is preferably used.

The lithium salt may be used without any limitation as long as it is commonly used in the art as an electrolyte salt used to increase ionic conductivity. Specific examples of the lithium salt include LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiC ₄ BO ₈ , LiAsF ₆ , LiSbF. ₆ , LiAlCl ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, (CF ₃ SO ₂ ) ₂ NLi, (C ₂ F ₅ SO ₂ ) ₂ NLi, (SO ₂ F) ₂ NLi, (CF ₃ SO ₂ ) ₃ One or more selected from the group consisting of CLi, chloroborane lithium, lower aliphatic lithium carboxylate having 4 or less carbon atoms, tetraphenylborate lithium, and lithium imide can be exemplified.

The concentration of the lithium salt can be determined by considering ionic conductivity, and is, for example, 0.1 to 2M, preferably 0.5 to 1M, more preferably 0.5 to 0.75M. Good too. If the concentration of the lithium salt is less than the above range, it may be difficult to ensure ionic conductivity suitable for battery operation, and if it exceeds the above range, the viscosity of the electrolyte increases and the mobility of lithium ions is reduced. or the decomposition reaction of the lithium salt itself may increase, resulting in a decrease in battery performance.

In the electrolyte containing the first solvent, second solvent, and lithium salt as described above, the molar ratio of the lithium salt, the second solvent, and the first solvent is 1:0.5 to 3:4.1 to 15. Good too. In one embodiment of the present invention, the molar ratio of the lithium salt, the second solvent, and the first solvent is 1:2:4 to 13, 1:3:3 to 10, or 1:4:5 to 10. The electrolyte contained in the lithium-sulfur battery of the present invention may contain a first solvent containing a fluorine-based ether compound at a higher content ratio than a second solvent containing a glyme-based compound. It is. In this way, when the first solvent containing a fluorine-based ether compound is contained in a higher content ratio than the second solvent containing a glyme-based compound, the formation of polysulfides is suppressed and the battery capacity approaches the theoretical capacity of sulfur. Since this is advantageous in terms of suppressing the decrease in battery capacity due to battery use, it is preferable that the first solvent containing the fluorine-based ether compound has a higher content ratio than the second solvent containing the glyme-based compound. It is preferable to set it so that it is included.

The negative electrode included in the lithium-sulfur battery of the present invention is a lithium-based metal, and may further include a current collector on one side of the lithium-based metal. The current collector may be a negative current collector. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and may include copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, It can be selected from the group consisting of these alloys and combinations thereof. The stainless steel can be surface treated with carbon, nickel, titanium or silver, the alloy can be an aluminum-cadmium alloy, as well as calcined carbon, non-conductive surface treated with a conductive material. A conductive polymer or a conductive polymer can also be used. Generally, a thin copper plate is used as the negative electrode current collector.

In addition, various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc., with or without fine irregularities formed on the surface can be used. In addition, the negative electrode current collector may have a thickness of 3 to 50 μm. If the thickness of the negative electrode current collector is less than 3 μm, the current collection effect will be reduced, while if the thickness exceeds 50 μm, the processability will be reduced when the cell is assembled by folding. There is.

The lithium-based metal may be lithium or a lithium alloy. At this time, the lithium alloy contains elements that can be alloyed with lithium, specifically, lithium and Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, It may be an alloy with one or more selected from the group consisting of Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, and Al.

The lithium-based metal may be in the form of a sheet or a foil, and in some cases, lithium or a lithium alloy may be vapor-deposited or coated on a current collector by a dry process, or particulate metals and alloys may be formed. The material may be deposited or coated using a wet process.

A conventional separation membrane may be interposed between the positive electrode and the negative electrode. The separation membrane is a physical separation membrane that has the function of physically separating the electrodes, and can be used without any particular restrictions as long as it is used in ordinary separation membranes. It is preferable to use a material that has a low resistance to moisture and has an excellent ability to moisturize the electrolytic solution.

Further, the separator separates or insulates the positive electrode and the negative electrode from each other, and allows lithium ions to be transported between the positive electrode and the negative electrode. Such separation membranes can be made of porous, non-conductive or insulating materials. The separator may be an independent member such as a film, or a coating layer added to the positive electrode and/or the negative electrode.

Examples of polyolefin-based porous membranes that can be used as the separation membrane include polyethylenes such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, ultra-high molecular weight polyethylene, polyolefins such as polypropylene, polybutylene, and polypentene. Examples include a film formed of a single polymer or a mixture of polymers. Examples of nonwoven fabrics that can be used as the separation membrane include polyphenylene oxide, polyimide, polyamide, polycarbonate, and polyethylene terephthalate. alate), polyethylene naphthalate, polyethylene naphthalate, butylene terephthalate, polyphenylene sulfide, polyacetal, polyether sulfone, polyether ether ketone Made of polymers such as herketone, polyester, etc., each alone or a mixture of these. Such nonwoven fabrics include spunbond or meltblown forms composed of long fibers as the form of fibers forming the porous web.

The thickness of the separation membrane is not particularly limited, but is preferably in the range of 1 to 100 μm, more preferably in the range of 5 to 50 μm. If the thickness of the separation membrane is less than 1 μm, mechanical properties cannot be maintained, and if it exceeds 100 μm, the separation membrane acts as a resistance layer, resulting in a decrease in battery performance. Although the pore size and porosity of the separation membrane are not particularly limited, it is preferable that the pore size is 0.1 to 50 μm and the porosity is 10 to 95%. If the pore size of the separation membrane is less than 0.1 μm or the porosity is less than 10%, the separation membrane acts as a resistance layer, and if the pore size exceeds 50 μm or the porosity is less than 10%, the separation membrane acts as a resistance layer. If the ratio exceeds 95%, mechanical properties cannot be maintained.

The lithium-sulfur battery of the present invention, which includes the positive electrode, negative electrode, separator, and electrolyte, is manufactured through a process in which the positive electrode faces the negative electrode, a separator is interposed therebetween, and then an electrolyte is injected. I can do it.

Meanwhile, the lithium-sulfur battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery of a battery module that is a power source for medium and large devices. In this aspect, the present invention also provides a battery module including two or more lithium-sulfur batteries electrically connected (in series or in parallel). Of course, the number of lithium-sulfur batteries included in the battery module can be variously adjusted in consideration of the usage and capacity of the battery module. Further, the present invention provides a battery pack in which the battery modules are electrically connected based on the conventional technology in the art. The battery module and battery packs are power tools; electric vehicles (ELECTRIC VEHICLE, EV), hybrid electrical vehicles (HYBRID ELECTRIC VEHICLE, HEV), and plug -in highbrid electric vehicles (PLUG). -In Hybrid Eleechnology Vehicle, PHEV ); electric trucks; electric commercial vehicles; However, the lithium-sulfur battery of the present invention is preferably an aircraft battery used in urban air mobility (UAM).

Hereinafter, preferred embodiments will be presented to help understand the present invention. However, the following embodiments are merely illustrative of the present invention, and various changes and modifications can be made within the scope of the scope and technical idea of the present invention. It is obvious to those skilled in the art that such changes and modifications are within the scope of the appended claims.

[Examples 1 to 8, Comparative Examples 1 to 4] Production of porous carbon material
After the porous carbon material was placed in a glass jar, nitrogen gas was injected and purging was performed for 1 minute, and then the porous carbon material was irradiated with microwaves. At this time, the porous carbon materials used in Examples 1 to 8 and Comparative Examples 1 to 4 were as follows. The microwave energy values (MPPT) received by each unit carbon material used in Examples 1 to 8 and Comparative Examples 1 to 4 are shown in Table 1 below.

- Example 1, Example 5, Comparative Example 1: Carbon nanotubes with a specific surface area of 270 m ² /g - Example 2, Example 6, Comparative Example 2: Multilayer with a specific surface area of 1,600 m ² /g Graphene flakes - Example 3, Example 7, Comparative Example 3: Carbon black with a specific surface area of 1,550 m ² /g - Example 4, Example 8, Comparative Example 4: Specific surface area of 1,350 m ² /g Ketjenbrack is

[Comparative Example 5] Porous carbon material
Carbon nanotubes having a specific surface area of 270 m ² /g, which were the same as those used in Example 1, Example 5, and Comparative Example 1, were prepared. That is, unlike Examples 1 to 8 and Comparative Examples 1 to 4, the carbon material was not irradiated with microwaves.

[Experiment Example 1] Evaluation of removal rate of impurities contained in porous carbon material
The content of impurities removed from each of the porous carbon materials prepared in Examples 1 to 8 and Comparative Examples 1 to 4 was measured, and the results are shown in Table 2 below. That is, it was confirmed how much the content of impurities contained in the carbon material was reduced after irradiation with microwaves. A precision scale (model: ML204T/00, manufacturer: Mettler Toledo) with a 0.1 mg measurement unit was used for the measurement.

As a result of measuring the content of impurities removed from each of the porous carbon materials manufactured in Examples 1 to 8 and Comparative Examples 1 to 4 as described above, as shown in Table 1 above, the unit carbon material It was confirmed that impurities were completely removed from the porous carbon materials of Examples 1 to 8, in which the microwave energy received by the porous carbon materials was set at 2,000 to 10,000 W*s/g. On the other hand, it was found that the removal of impurities was insufficient in the porous carbon materials of Comparative Examples 1 to 3 in which the microwave energy value received by the unit carbon material was set at less than 2,000 W*s/g. In addition, in the case of Comparative Example 4 in which the microwave energy value received by the unit carbon material was set to exceed 10,000 W*s/g, impurities were completely removed as in Examples 1 to 8, but ignition occurred. also occurred together.

[Experiment Example 2] Confirmation of impurity removal through thermogravimetric analysis (TGA)
In order to confirm whether or not impurities were removed from the porous carbon material produced in Example 1, TGA analysis (10°C/min temperature increase condition (R .T~500°C), nitrogen atmosphere).

FIG. 1 is a TGA analysis graph for confirming whether impurities are removed from porous carbon materials according to an example of the present invention and a comparative example. As a result of TGA analysis of the porous carbon material manufactured in Example 1 and the porous carbon material of Comparative Example 5, the microwave energy value received by a unit carbon material was set at 2,000 to 10,000 W*s/g. As shown in FIG. 1, the porous carbon material of Example 1, which was subjected to the irradiation, showed significantly less weight loss due to temperature increase than the porous carbon material of Comparative Example 5, which was not irradiated with microwaves. I was able to do that. Therefore, it can be seen that impurities such as moisture are removed by applying microwave energy of 2,000 to 10,000 W*s/g to the porous carbon material (on the other hand, in Figure 1, the weight decreases up to 100°C). The reason for this is that moisture is absorbed during storage of carbon materials).

[Experiment Example 3] Confirmation of impurity removal through elemental analysis (EA)
In order to confirm whether or not impurities were removed from the porous carbon material manufactured in Example 1, the porous carbon material of Comparative Example 5 (untreated with microwave) was used with an elemental analyzer (model: Flash 2000, manufacturer: Thermo EA analysis was performed using Scientific ^™ ) and the results are shown in Table 3 below.

As a result of EA analysis of the porous carbon material manufactured in Example 1 and the porous carbon material of Comparative Example 5, the microwave energy value received by a unit carbon material was set at 2,000 to 10,000 W*s/g. It was confirmed that the carbon (C) content of the porous carbon material of Example 1 was increased by removing impurities such as moisture.

[Comparative Example 6] Production of sulfur-carbon composite
The carbon nanotubes of Comparative Example 5 that were not subjected to microwave treatment and sulfur (S) were mixed at a weight ratio of 75:25 and then dried to prepare a sulfur-carbon composite, and then the prepared sulfur- After the carbon composite was placed in a glass jar, nitrogen gas was injected and purging was performed for 1 minute, followed by microwave irradiation (MPPT: 2,160 W*s/g).

[Experiment example 4] Temperature profile evaluation based on microwave irradiation time
While the porous carbon material manufactured in Example 1 and the sulfur-carbon composite manufactured in Comparative Example 6 were irradiated with microwaves (MPPT: 2,160 W*s/g), the temperature was changed over time. Each was measured with a Thermocouple (306 data logger, Conrad Electronics, Hirschau, Germany), and the results are shown in FIG. FIG. 2 is a graph showing a temperature profile over time when a porous carbon material and a sulfur-carbon composite are irradiated with microwaves under the same conditions.

The temperature of the porous carbon material of Example 1 and the sulfur-carbon composite of Comparative Example 6 was measured every 5 seconds while irradiating microwaves. As shown in FIG. It was confirmed that the temperature increase rate of Example 1) was much faster than that of the sulfur-carbon composite (Comparative Example 6). In other words, when irradiated with microwaves, the temperature of carbon rises faster than sulfur, so the temperature of carbon rises first, and the energy generated when the temperature of carbon rises is transferred to sulfur, causing sulfur to vaporize. It is converted into energy, and at this time, sulfur is vaporized from the low temperature before the impurities contained in the carbon are removed.

In other words, when microwave irradiation is performed under the same MPPT conditions, in the case of the porous carbon material (Example 1), the temperature rises to approximately 400°C based on the point at which 30 seconds have elapsed since microwave irradiation. impurities are removed. On the other hand, the sulfur-carbon composite (Comparative Example 6) was heated to about 200°C (temperature at which sulfur volatilization occurs rapidly) after 30 seconds of microwave irradiation, and sulfur was volatilized along with impurities in the carbon material. . Therefore, if microwaves were applied to the sulfur-carbon composite (Comparative Example 6) itself, it would not only be impossible to selectively remove only the impurities within the carbon material, but it would also warp and cause loss of sulfur. This has no choice but to adversely affect battery performance. Therefore, it can be seen that even if microwaves are applied to the carbon material, the object of the present invention can be achieved by applying microwaves only to the carbon material itself, which is not in a composite state with sulfur.

[Experiment Example 5] Thermogravimetric analysis (TGA)
In Experimental Example 2, it was confirmed through thermogravimetric analysis (TGA) that impurities such as water were removed from the porous carbon material of Example 1. For comparison and contrast, the sulfur-carbon composite of Comparative Example 6 was also subjected to TGA analysis (10°C/min temperature increase condition (RT~500°C), nitrogen atmosphere). carried out.

FIG. 3 is a TGA analysis graph showing the degree of weight loss due to temperature increase obtained by performing TGA analysis on a porous carbon material according to an embodiment of the present invention and a sulfur-carbon composite according to a comparative example. As a result of TGA analysis of the porous carbon material of Example 1 and the sulfur-carbon composite of Comparative Example 6, as shown in FIG. It can be confirmed that the weight loss due to temperature increase is significantly less than that of the composite. As explained in Experimental Example 4 above, even if the porous carbon material (Example 1) and the sulfur-carbon composite (Comparative Example 6) were irradiated with microwaves under the same MPPT conditions, the sulfur-carbon This is due to the fact that in the composite (Comparative Example 6), sulfur is volatilized along with impurities in the carbon material.

In other words, even through the results of this experiment, it is not possible to selectively remove only the impurities contained in the carbon material in the sulfur-carbon composite, and the impurities can only warp and cause loss of sulfur, which has a negative effect on battery performance. I can confirm that it is not. Therefore, it can be seen that even if microwaves are applied to the carbon material, the object of the present invention can be achieved by applying microwaves only to the carbon material itself, which is not in a composite state with sulfur.

Claims (15)

A porous carbon material having a specific surface area of 200 m ² /g to 1,700 m ² /g and having impurities removed through pretreatment using microwaves. The pretreatment using microwaves is characterized in that microwaves are applied to the porous carbon material under the condition that MPPT is 2,000 W*s/g to 10,000 W*s/g according to the following formula 1. Porous carbon material according to claim 1:
[Formula 1]
MPPT (W*s/g) = Microwave Power (W) x Time (s)/Carbon material mass (g)
In Equation 1, the time (Times) exceeds 10 seconds as the time in seconds during which the microwave is applied. The porous carbon material is selected from the group consisting of carbon nanotubes; multilayer graphene flake (MGF); graphite; carbon black, acetylene black, Ketjen black, Denka black, thermal black, channel black, furnace black, and lamp black. The porous carbon material according to claim 2, characterized in that the porous carbon material is selected from the group consisting of carbon black; carbon fiber; and a mixture containing two or more of these. The porous carbon material according to claim 3, wherein the porous carbon material is selected from the group consisting of carbon nanotubes, multilayer graphene flakes (MGF), carbon black, and Ketjenblack. The porous carbon material according to claim 1, wherein the porous carbon material has a pore volume of 1.5 cm ³ /g or more. The porous carbon material according to claim 1, wherein the porous carbon material has 90% to 100% of the total impurities contained in the porous carbon material removed. 7. The porous carbon material according to claim 6, wherein 99% to 100% of all impurities contained in the porous carbon material have been removed. The porous carbon material according to claim 1, wherein the impurities include moisture and functional groups present inside and on the surface of the carbon material. (a) putting a porous carbon material with a specific surface area of 200 to 1,700 m ² /g into a closed container and then purging it by injecting an inert gas; and (b) applying microwaves to the porous carbon material. A method for producing a porous carbon material from which impurities have been removed, the method comprising: applying an impurity. 9. The step (b) is characterized in that microwaves are applied to the porous carbon material under the condition that MPPT is 2,000 W*s/g to 10,000 W*s/g according to the following equation 1. Method for producing porous carbon material from which impurities are removed:
[Formula 1]
MPPT (W*s/g) = Microwave Power (W) x Time (s)/Carbon material mass (g)
In Equation 1, the time (Times) exceeds 10 seconds as the time in seconds during which the microwave is applied. A positive electrode for a lithium-sulfur battery comprising, as a positive electrode active material, a sulfur-carbon composite in which sulfur is supported on a porous carbon material from which impurities have been removed according to claim 1. The positive electrode for a lithium-sulfur battery according to claim 11, wherein there is no error in the content of sulfur and carbon material contained in the positive electrode active material. A positive electrode for a lithium-sulfur battery according to claim 11; a negative electrode; a separation membrane interposed between these; and a first solvent containing a fluorine-based ether compound, a second solvent containing a glyme-based compound, and an electrolyte containing a lithium salt. Lithium-sulfur batteries, including; The lithium-sulfur battery according to claim 13, wherein the utilization rate of sulfur contained in the positive electrode is 90% or more of the theoretical discharge capacity. The lithium-sulfur battery according to claim 13, wherein the lithium-sulfur battery has an energy density of 400 Wh/kg or more or 600 Wh/L or more.